Previous power systems for providing protected (i.e., uninterrupted) power have generally been designated as either series processing systems or parallel processing systems. In the series processing system the primary AC voltage is rectified into a DC voltage which is applied to float a battery. The DC voltage energizes an inverter connected in series with the rectifier battery combination. DC voltage to the inverter input is continuous from either the battery or rectifier, hence the output voltage is truly uninterrupted. This advantage is counterbalanced by the low efficiency (e.g., 76.5%) of the series protected power system and in addition high cost and large size disadvantages, especially at very low frequencies.
Another widely used protected power system is the parallel processing protected power system. In this system both primary AC voltage and the AC output of a DC voltage (i.e., a battery) powered inverter are coupled to a load, for example, through a transformer with the turns ratio adjusting the voltage to a desired output voltage level. With system components connected in parallel, the resulting operating efficiency is quite high (e.g., 85% to 90%). A critical disadvantage is the time lost (i.e., transfer time) in transferring the load from the primary (i.e., AC line) AC voltage to the (i.e. inverter) reserve AC voltage. Typically the inverter is activated only upon failure of the primary AC voltage and hence load transfer is delayed. Even when the inverter is continuously operating in an idling mode, the transfer time is often unacceptable in communication system applications and system fixes to accommodate this transfer time are expensive. It is further disadvantaged by being operative at line frequency only.
In both instances the large size of the magnetic components at low frequencies makes their use impracticable for most applications and especially for applications operating at low frequencies.
A typical exemplary series processing type protected power system is shown in FIG. 1. AC line power is applied to the input 101 of the bulk and battery charge rectifier package 102 whose output is connected in series to an inverter 103 and is also applied to float the battery 104 supplying reserve energy. The DC voltage of the battery 104 energizes the inverter 103 upon failure of the commercial line AC voltage.
The exemplary parallel processing type protected power system of FIG. 2 applies commercial AC line voltage to two leads 201 and 202 connected in parallel to an AC power conditioner 205, via a transfer switch contact 203, and to a charging circuit 206, respectively. The charging circuit maintains a battery 207, at a float voltage and the battery 207 is connected to the input of an inverter 208 via a transfer switch contact 209. Transfer switch contacts 203 and 209 are controlled by a mechanical transfer switch control 211 which monitors the commercial AC voltage, via lead 212. As long as the AC voltage is satisfactory the switch 203 is closed and switch 209 is open thereby enabling direct application of the AC voltage to the AC power conditioner 205.
The AC power conditioner 205 comprises a ferroresonant transformer with two isolated inputs, one from lead 201 and the other from the inverter 208. The power conditioner may be a ferroresonant transformer which couples AC line energy and inverter output to the AC output 210 which is connected to a load. The AC commercial AC voltage is the primary power source and upon its failure power for the AC load is derived from the battery voltage.
Neither of these two protected power systems is suitable for low frequency power systems (for example one Hertz) which are suitable for certain communication systems. Such low frequencies are advantageous in reducing corrosion in the wire distribution system.